1. Field of the Invention
The invention generally relates to water-repellent metal organic framework (MOF) molecules and methods for synthesizing such MOFs, such as, for example microwave assisted synthesis.
2. Related Art
MOFs are organometallic nanoporous structures with high surface area and tailorable selectivity. MOFs may have a cubic crystalline structure that is formed by copolymerization of metals or metal oxides with organic ligands, resulting in metal-oxide clusters connected by organic linkers. FIG. 1 is a diagram of a typical MOF's crystalline structure 10 including metal or metal oxides, here shown as polyhedrons 12, having polymer ligands 14 extending between them. This highly ordered structure facilitates the creation of interior pores and channels. MOFs are known to have about 0.3 nm to about 3 nm pores.
MOFs are thermally robust and in many cases have extremely high porosity. Potential applications for MOFs include gas storage, adsorbents, and catalysts as described in detail in U.S. application Ser. No. 11/539,405, which is expressly incorporated by reference herein in its entirety. Applicants have discovered that certain MOFs have properties that make them highly advantageous as preconcentrators of analytes, including, for example, a high sorption capacity due to their high surface area, a high selectivity to specific analytes, an inert nature which does not decompose the analyte, a thermal stability, which result in unexpectedly high gains in detection, and as further described in U.S. application Ser. No. 11/539,405. Accordingly, MOFs are used to selectively sorb specific analytes in a preconcentrator. MOFs may be used in particle or pellet form, or they may be incorporated into a film inside a preconcentrator. Once the analytes are fully sorbed by the MOFs, the analytes can be released, for example, by thermal desorption. The analytes can then be purged and transferred from the preconcentrator to a detector.
One disadvantage associated with currently available MOFs is their lack of stability and resultant decrease in surface area when exposed to environmental conditions having greater than about 4% water present. Indeed, studies have indicated that water molecules attack the coordination bonds between the metal and organic ligands. Thus, MOF applications may be adversely affected under the most common environmental conditions.
One way to overcome this disadvantage is to fabricate MOFs by incorporating water repellent functional groups onto the ligand to increase the stability of the MOF when exposed to environmental conditions having greater than about 4% water present. This may be accomplished by building porous frameworks with covalent bonds using well-defined organic ligands. But, due to the high reactivities of the organic ligands, their synthesis requires complex processes and demanding crystallization conditions; thus, making this synthesis method undesirable. As an alternative, the MOF framework may be built with ligands using coordinate bonding. Although coordination bonding is not as strong as covalent bonding, it requires milder conditions to create the framework and offers a larger variety of building blocks (e.g., terephthalic acid with different functional groups) that can be used to build the framework relative to building the framework using covalent bonds.
MOF frameworks using coordination bonds may be synthesized using either a simple solvothermal, microwave-assisted solvothermal, or hydrothermal synthesis method, for example, as disclosed in Applicants' application Ser. No. 11/785,102, which is expressly incorporated by reference herein in its entirety. Solvothermal synthesis is a method where ligands for MOF crystal formation are heated in a solvent other than water at high vapor pressure. In hydrothermal synthesis, ligands for MOF crystals are heated in water. Hydrothermal synthesis is suitable when the ligand is soluble in water. In both conventional solvothermal and hydrothermal synthesis, a solution with MOF ligands is typically maintained at a predetermined equilibrium temperature and pressure for an extended period to induce crystallization.